You might be referring to the following verse:
فَبِأَيِّ آلَاء رَبِّكَ تَتَمَارَى
“Then which of the favors of your Lord do you doubt?” (An-Najm/The Star 53:55)
The verse indicates that we can’t doubt the blessings of God.
“Doubt” has two types. One occurs during a research in a form of query, which helps a person to discover the truth. The other one forces a person to distrust although the truth reveals itself. The second one conveys the state of a concealer’s mind. Here are some verses about the subject:
“And do not be like those who separated and disputed after the solid evidences came to them; for them is a great chastisement.On the Day [some] faces will turn white and [some] faces will turn black. To those whose faces turn black, [it is said], “Did you conceal (the truth) after your belief? Then taste the punishment for what you used to conceal.” But those whose faces will turn white, will be within the beneficence of Allah. They will abide therein. These are the verses of Allah. We recite them to you, [O Muhammad], in truth; and Allah wants no wrong-doing for the worlds.” (Al-e-Imran/Family of Imran 3:105-108)